1. Field of the Invention
The present invention relates to a magenta color liquid developer used for image formation in the electrophotographic process, and more particularly, a magenta color liquid developer comprising a toner component which is electrostatically deposited on a latent electrostatic image formed on an electrophotographic photoconductor or on an electrostatic recording medium, thereby developing the latent electrostatic image into a visible toner image.
2. Discussion of Background
To obtain a multicolor image by an electrophotographic method or electrostatic recording method, a latent electrostatic image is first formed on an electrophotographic photoconductor or on a sheet of electrostatic recording paper by the conventional methods. The above-mentioned latent electrostatic image is developed into a visible toner image by color liquid developers in accordance with the subtractive color mixture. Namely, a liquid developer of one primary color is electrostatically deposited on the latent electrostatic image. Another latent electrostatic image is subsequently formed, followed by developing by a liquid developer of another primary color, and the same step as mentioned above is repeated with respect to other colors, so that the latent electrostatic images are developed into a multicolor image.
The liquid developers of primary colors, that is, a cyan liquid developer, a magenta liquid developer and a yellow liquid developer, are separately prepared as follows: The respective pigments are ground and dispersed in a carrier liquid such as an aliphatic hydrocarbon with excellent insulating properties and a low dielectric constant, together with a resin, for example, rosin, linseed oil, soybean oil, modified alkyd resin, styrene-butadiene resin and acrylic resin, and fats and oils.
In multicolor electrophotography, as previously mentioned, a multicolor image can be obtained by subsequently overlapping the development by a plurality of developers of the primary colors. Those liquid color developers are therefore required to have various properties, in particular, required to achieve faithful color reproduction.
Generally, the conventional magenta color liquid developer, one of the liquid developers of primary colors, comprises azo-lake pigments, represented by Carmine 6B. This is because the azo-lake pigments are capable of producing images with satisfactory color tone and high transparency. All the toners comprising the azo-lake pigments, however, have the shortcoming that they readily induce fogging in the background.
When a single color image is formed without overlapping the developers of other colors, the color tone of an image can generally be improved by the addition of a specific dye to a specific pigment in the employed developer. According to the above method for improving the color tone, however, fogging readily takes place as an adverse side effect of the additional dye.
To improve the color tone of a magenta color image, a mixture of a rhodamine-type pigment and a quinacridone-type pigment is contained in a magenta developer as disclosed in Japanese Laid-Open Patent Application 56-75660. By using such a magenta developer, a magenta color image can be obtained with a color tone remarkably close to the ideal magenta color tone.
When the secondary color is produced by overlapping the development of a magenta color and other colors, the color produced by the magenta color developer is occasionally required to assume a red tinged magenta color or a blue tinged magenta color as compared with the above-mentioned ideal magenta color depending upon the overlapping order of colors and the color tone of the other colors. It is therefore difficult to use the aforementioned magenta developer comprising a mixture of pigments for producing the secondary color faithfully by overlapping the development of a plurality of other color developers because the controllable range of color tone, that is, a metric hue-angle, of the above magenta color developer is narrow for use in practice. Furthermore, the aforementioned rhodamine-type pigment contained in the magenta color developer is so poor in light resistance that the tone of the obtained color image changes with time.
To solve the aforementioned fogging problem, a Naphthol AS-type water-insoluble azo pigment, a benzimidazolone-type water-insoluble pigment and a .beta.-naphthol type water-insoluble pigment are proposed for use as the pigments for the toner composition of a magenta color, as disclosed in Japanese Laid-Open Patent Applications 64-2065, 01-94353 and 01-116565, respectively.
In the case where the magenta liquid developers comprise one of the above-mentioned pigments, images can satisfactorily be produced free from the fogging problem and other abnormality. However, the color tone of such a magenta liquid developer comprising one of the above pigments is determined by the individual color of the pigment, so that it is extremely difficult to change the color tone of a magenta color as desired. Consequently, faithful color reproduction cannot be achieved by superimposing other colors on that kind of magenta color depending upon the selection of liquid developers of other colors.